In DWM (what I have now read. mostly), when asked about this, Moffat states that its because he likes Superhero films to watch, but he couldn't write them (insert your own joke here). He mentions that like Brendocon, there's something very silly that no one can tell Clark Kent is Superman just because he wears glasses. Turns out Lois And Clark was an influence on this, and it was relationship stuff which he remembered he was pretty god at with Coupling, so managed to find a way to write it.